If you are creating a iOS Signing Certificate after January 28, 2021, then these new signing certificates have a newer Certification Authority (CA) certificate. If you do not have this CA certificate, then your will certificate will show in Keychain Access that it is not trusted.
The new CA certificate can be downloaded from:
https://www.apple.com/certificateauthority/AppleWWDRCAG3.cer
Open the certificate and it will be added to the keychain. Then your certificate will show as trusted.
